Method for determining the extent of the burnt zone in an underground combustion process by passing current around the boundary of the zone



Juiy 4. 1967 j c. TODD 3,329,891

METHOD FOR DETERMINING T E: EXTENT OF THE BURNT ZO E IN AN UNDERGROUND COMBUSTION PROCESS BY PASSING CURRENT AROUND THE BOUNDARY OR'TRE ZONE Filed April 5, 1965 FIG. I

RADIAL DISTANCE v v H 36 FIG. 2 r 201/ 22 I v GENERATOR 24 v i 1. J22 J N v 1) A I v Ill FIG. 3

INYENTOR JOHN c. moo

ATTORNEYS.

METHOD FOR DE ERMINING THE EXTENT OF THE BURNT zONE IN AN UNDERGROUND coMBUsrioN rnocnss BY PASSING CUR- REgT AROUND THE BOUNDARY on THE an a John C. Todd, Tulsa, Okla, assiguor, by mesne assign- United States Patent ments, to Sinclair Research, Inc., New York, N.Y., a

corporation of Delaware Filed Apr. 5, 1965, Ser. No. 445,679 5 Claims. (Cl. 324-) This invention relates to the recovery of hydrocarbon materials from a subterranean formation by a method involving combustion in place of a portion of the hydrocarbons in the subterranean formation, and more particularly, the present invention relates to a method for locating the flame front of such combustion in the subterranean formation.

It has been proposed to recover hydrocarbon materials from a hydrocarbon-containing subterranean formation by a method which involves combustion of a portion of the hydrocarbons within the formation. In this method, a combustion supporting gas such as air is injected into the formation through an input well and combustion of hydrocarbon within the formation is initiated by suitable means. The formation is, for example, provided with a.

single output well, or more, if desired, e.g. with four output wells each spaced 90 from each other on a circle having an input well as its center; etc., and, as the flow of combustion supporting gas to the formation is continued, a flame front migrates from the input well to the output well or output wells. Combustion gases, oil, and distillation and viscosity breaking products of the hydrocarbon migrate in advance of the flame front to the output well or wells from which they are removed to be treated for recovery of the desired valuable hydrocarbon material or other constituents. The heat of the ice Several methods for finding the flame front or the area swept by the flame are known. For example, mathematical and potentiometric models have been used to find the flame front or region. These are however, attended by many assumptions and simplifications. Changes in the magnetic field due to alteration of the magnetic properties of in situ materials by heat have been used in a method to find the swept area. This method is considered insensitive in most formations due to little or no alteration in magnetic properties with the passage of the heat wave. Electrical prospecting methods have also been attempted; however, the currents applied from the surface are deflected so little by the relatively small volume of the non-conductive swept areas as to render the method insensitive except in formations where the depth is perhaps less than 200 feet. 7

In view of the foregoing, it is an object of this invention to provide, in a process of underground combustion, an efficient method of following the combustion front as it progresses through a reservoir so that, for example, the rate of oxidant injection can be regulated to the necessary value with some precision. Another object is to provide a method of measurement of the burned-out area in an underground combustion project, which methodmay be carried out at any time desired. A further object is to provide a method of utilizing, for the measurement of area of an undeground combustion zone, certain electrical properties of the rock which are effected by fluids migrating in advance of the flame front strips the hydrocarbon-containing formation of water and the greater portion of the hydrocarbon leaving behind a carbonaceous deposit which is essentially the fuel consumed in the process.

In carrying out this combustion recovery method, it is often necessary or desirable to determine the location of the flame front with respect to its distance between theinput well and the output well or one or more of the output wells, for example, in order to estimate the v yield of hydrocarbon materials, to estimate the time of arrival of the flame front at an output well, to control the rate of advance of the flame front, or to effect other purposes. One particular difiiculty encountered in the field operation of a combustion process is in determining the necessary amount of oxygen or other combustion supporting gas required to maintain the combustion front in motion with at least a minimum velocity. Lacking exact knowledge of the front position at any given time, expensive additional equipment is required to provide a margin of safety that allows for errors in estimating the front position on the basis of general engineering data. It has been recognized that, if the exact position of the front can be ascertained at any desired time, the oxygen input rate can be maintained much closer to the absolute minimum value required for successful operation, thereby controlling the character of the combustion to some extent as well as reducing the equipment investment required by a substantial fraction, particularly when the dimensions of the front become very large. Furthermore, to obtain and control an eflicient sweep of the formation in a heat wave process for oil recovery itis desired to define the limits of the sweep pattern. By so defining the swept areas, the injection and production from the various wells may be controlled to affect the desired sweep.

the passage of the combustion front. Other objects, uses and advantages will become apparent as the description of the invention proceeds.

In accordance with this invention, the foregoing objects are accomplished by an electrical method comprising a series of steps which may preferably, but will not necessarily, include as a preliminary step, prior to the beginning of the operation, the installationof most or all of the surface and well equipment to be used in the injection and producing wells, including the drilling of the well bore and emplacement of the well casing. The method is directly applicable to well bores cased to the top of the formation. If the method is to be applied to cased through holes, which would give rise to an electrical short from above to below the burnt formation through the casing, the casing must be severed in the middle of the formation. The casing can be severed by standard cut-off tools of various sorts known in the industry and thus the method is also applicable in wells cased through the formation.

Following installation of the equipment, combustion is initiated at the base of the injection well or wells and with introduction of combustion supporting gas at the proper rate and concentration, a combustion zone or front is caused to propagate through the reservoir away from each injection well. Subsequently, at any time during the operation, and preferably whenever exact information on the location of the underground combustion front is needed the method of this invention is effected.

In general, the method of this invention for determining the extent of burning comprises passing an AC constant current from the Well casing immediately above the formation to an electrode contacting the formation just below the swept zone. Since the swept portion, e.g. burnt area, is practically non-conductive, the current must follow a path around the outer boundary of the swept portion of the formation. Thus, an alternating field will be established in the area above the swept zone which can be received and measured by a meter. or other display device on the surface ofthe ground and the field strength thus found plotted versus the radial distance from the well. The strength of this alternating field. decreases gradually with the radial distance and then deflects sharply to alower value over the outer boundary of the flection point distinguishes the outer boundary of the swept area in the direction in which the measurement took place and similar measurements along different radial directions will produce a plot defining the limits of the swept portion, e.g. burnt area.

The present invention will be better understood by reference to the accompanying drawing forming a part of this application, wherein FIGURE 1 shows diagrammatically and partly in cross-section a portion of the earth containing an oilbearing stratum penetrated by an oxygen input well surrounded by a combustion area. Superimposed above this cross-section with the same horizontal scale is a plot of field strength versus radial distance in the underground stratum for correspondingly marked different com-bustion-front positions shown in the stratum;

FIGURE 2 shows schematically, partially in crosssection, a detail of a combustion area and apparatus for use in the present invention; and

FIGURE 3 is a detailed view of the electrode which is attached to the well casing.

Referring now to FIGURE 1, an input well extends down from the earths surface 11, penetrating sub-surface layers 12 and terminating in an oil-bearing stratum 14. As the particular arrangement of oxygen-injecting and oil-recovery equipment for the input and output wells is not important in the present invention, only a single input well 10 is shown. Oxygen injection is begun through well 10, ignition of the oil in place at the well bottom is accomplished, and the combustion front 16 is propagated through the formation 14 radially outward from well 10 in the direction of the arrows according to conventional practice. The rate of propagation of the front 16 from well 10 is estimated as closely as possible from a knowledge of the amount of oxygen injected and of the combustion characteristics of the oil and reservoir rock. At such times, however, as it is desired to ascertain more exactly the area which has been covered by the underground combustion front, a survey is performed in accordance with this invention with the apparatus illustrated in FIGURES 2 and 3 which can be installed in well 10 before combustion begins or at a later time, if desired.

Referring now to FIGURE 2, the apparatus comprises a pair of shielded cables, i.e. conductors, 20 and 22, in a circuit with a constant current generator 24. One of the cables, i.e. cable 20, is connected to an electrode 26 which is in electrical contact with the formation of earth beneath the burnt zone of the stratum 14 and the other cable, i.e. cable 22, is in contact with an electrode 28 which is attached to the well casing 30, and consequently the earth, above--the burnt zone 15. Contact of electrode 26 with the earth is insured by placing the electrode in a body 32 of salt water or mercury provided field strength decreases gradually with the radial distance and then deflects sharply to a lower value over the outer boundary of the area contacted by the heat wave. The deflection point distinguishes the boundary of the swept area eg burnt zone 15, in that direction to define a limit of the burnt volume. Since the alternating field will extend 360 degrees about the well (the well casing is conductive), a map or outline of the swept area, e.g. burnt zone, is provided by measuring the field in several radial directions from well-10. In this invention, it is not absolute values of field strength that are important, but the deflection points. If the flame front is measured at point A1 in FIGURE 1, the field strength vs. radial distance will have the general configuration illustrated with deflection point a1 locating the flame front. Determination of a flame front at point B1 can require greater field strength adjacent the casing in order to provide a detectable deflection point at [:1 due to the greater radial at the bottom of well 10. Electrode 28 is attached to the casing 30 by electromagnet 34, see FIGURE 3, which is controlled by switch 36.

In operation, an AC constant current is passed through cables 20 and 22. The current follows the path, represented by lines 38, from the electrode 28 immediately above the formation or stratum 14 to electrode 26 contacting the formation just below the swept zone 15. Since the swept zone 15 is practically non-conductive, the current must follow path 38 around the outer boundary of the swept portion of the formation as shown in FIG- URE 2. Thus, an alternating field is established in the area above the swept zone 15. This alternating field is then induced into a pick-up coil 42 of detecting device 40 which is tuned to the AC frequency of the established field. Several such devices for the detection and measure' ment of such an electrical field are known to those skilled in the art. The strength of this field is plotted versus the radial distance from the well as in FIGURE 1. This distance involved. Determination of deflection point a2, or b2, give the flame front position with respect to well 10 fr two directions.

The principal advantage of this method liesv in the concentration of the current around the conductance anomaly formed by the heated out zone. This concentration of the current flux around the anomaly gives a sensitive method of finding the swept areas. The AC current source may be of any convenient frequency though the very high frequencies may not be as applicable because of capacity effects. Also, although AC is considered the more useful current source, because of its ease of detection and filtering, it is recognized that similar results may be found with the use of a DC source.

The following example will be illustrative of the invention:

An input well and four output wells spaced from each other on a circle having the input well as its center, i.e., a five-spot well pattern, are drilled into a petroleum oil formation. Air is injected into the formation through the input well until the pressure at the out-put wells stabilizes. Combustion is then initiated within the forma tion and air is continuously injected into the formation through the input well to maintain the combustion. After a period of continued combustion, when it is desired to know the location of the flame front between the input well and well number one, two insulated cables each having an electrode on the end thereof are lowered into the input well and arranged one above the formation and one below the formation by conventional techniques. Thereupon, an AC constant current of sufiicient strength is passed from the electrode above the formation to the electrode below the formation and the field strength is measured on the surface by conventional techniques along a path between the input well and well No. 1. The strength of this field decreases gradually with distance from the input well and then deflects sharply. This point of sharp deflection is above the outer boundary of the burnt or swept area of the formation and locates the flame front.

It is claimed:

1. In the recovery of oil from an underground reservoir stratum by combustion of a portion of the oil in place in said stratum radially outwardly from an input well, a method for determining the radially outward extent of the burnt zone in said stratum utilizing a pair of electrodes in circuit with a current generator and an electric field detector, comprising the steps of locating the first of the pair of said electrodes in said well in electrical contact with the earth above and close to said burnt zone, locating the second of said pair of electrodes in the well in electrical contact with the earth beneath said burnt zone; passing an electric current through the earth between said electrodes and around the outer boundary of said burnt zone to concentrate the current at said zone whereby an electrical field is established above said zone, and detecting and measuring the strength of the resultant electric field above said zone along a path extending outwardly from the well, the outer boundary of said burnt Zone being characterized by a relativelysharp deflection in said field strength as it is measured along said path extending outwardly from the well.

2. The method of claim 1 wherein said electric current is an AC current.

3. The method of claim 1 wherein said detecting and measuring is carried out above ground level.

4. In the recovery of oil from an underground reservoir stratum by combustion of a portion of the oil in place in said stratum radially outwardly from an imput well, a method for mapping the outer boundary of the burnt zone in said stratum utilizing a pair of electrodes in circuit with a current generator and an electric field detector, comprising the steps of locating the first elec trocle of said pair of electrodes in said well in electrical contact with the earth above and close to said burnt zone, locating the second of said pair of electrodes in the Well in electrical contact with the earth beneath said burnt zone; passing an electric current through the earth between said electrodes and around the outer boundary of said burnt zone to concentrate the current at said zone whereby an electrical field is established above said zone, and separately detecting and measuring the strength of the resultant electric field above said zone along a pluralitypf separate paths extendingjn difierent directions outwardiy from the well, theouter boundary of said burnt zone being characterized by a relatively sharp deflection in said field strength as it is measured along each said path extending outwardly from the well, a plot of the deflection points obtained along said paths with reference to the well providing a map of the outer boundary of the burnt zone.

WALTER L. CARLSON, Primary Examiner.

G. R. STRECKER, Assistant Examiner. 

1. IN THE RECOVERY OF OIL FROM AN UNDERGROUND RESERVOIR STRATUM BY COMBUSTION OF A PORTION OF THE OIL IN PLACE IN SAID STRATUM RADIALLY OUTWARDLY FROM AN INPUT WELL, A METHOD FOR DETERMINING THE RADIALLY OUTWARD EXTENT OF THE BURNT ZONE IN SAID STRATUM UTILIZING A PAIR OF ELECTRODES IN CIRCUIT WITH A CURRENT GENERATOR AND AN ELECTRIC FIELD DETECTOR, COMPRISING THE STEPS OF LOCATING THE FIRST OF THE PAIR OF SAID ELECTRODES IN SAID WELL IN ELECTRICAL CONTACT WITH THE EARTH ABOVE AND CLOSE TO SAID BURNT ZONE, LOCATING THE SECOND OF SAID PAIR OF ELECTRODES IN THE WELL IN ELECTRICAL CONTACT WITH THE EARTH BENEATH SAID BURNT ZONE; PASSING AN ELECTRICAL CURRENT THROUGH THE EARTH BETWEEN SAID ELECTRODES AND AROUND THE OUTER BOUNDARY OF SAID BURNT ZONE TO CONCENTRATE THE CURRENT AT SAID ZONE WHEREBY AN ELECTRICAL FIELD IS ESTABLISHED ABOVE SAID ZONE, AND DETECTING AND MEASURING THE STRENGTH OF THE RESULTING 